1. Field of the Invention
This invention relates generally to the field of measuring the thickness of flat-rolled sheet material in a rolling mill. More specifically, this invention relates to a non-contact radiation attenuation gauge which is insensitive to variations in alloy composition.
2. Related Art
In the manufacturing of flat-rolled metal materials such as aluminum, steel, brass, copper, and stainless steel, a critical concern is meeting delivery specifications within very strict tolerances while maintaining a high production rate. Flat-rolled materials which are too thick raise manufacturing material costs and materials which are too thin have to be reworked to meet the desired specifications.
Most rolling mills have some form of control system for controlling the thickness of its flat-rolled products. These control systems must be able to control the rolling mills in times of approximately 0.1 seconds or less and are required to gauge thickness with an accuracy of approximately +/-0.25%. These systems have utilized either one of two gauging techniques: contacting mechanical gauge or non-contacting radiation gauge techniques.
Contacting mechanical thickness measurement techniques are undesirable because the sensor elements which are in contact with the material may score or mark the material. In addition, the sensor element may skip or bounce when the sheet material reaches high sheet speeds. This results in inaccurate measurements or damage to the sensor if the sheet material should flutter.
Non-contacting radiation gauges such as radioisotope, beta-ray, x-ray, and gamma ray gauges are more prominent today. However, some of these systems have also had difficulty in meeting the high measurement and time tolerances required. The accuracy of radiation gauges is affected by material and environmental conditions including air gap temperature and distance, the presence of rolling solution or oil on the material, passline height variations, material composition variations, and sheet flutter.
Beta-ray gauges, which are used in some sheet and foil mills, have been known for their relative non-responsiveness to alloy composition variations. However, because the beta gauge utilizes high energy electrons of discrete mass to measure the density of the material, the beta ray is sensitive to the density of any materials in its path. This makes the beta gauge impractical for rolling mill operations where the beta ray's sensitivity to air density, rolling mill solutions, and oil film reduces its accuracy. This sensitivity to air density requires the air gap distance to be relatively small, bringing the beta-ray source and detector in close proximity to the sheet material. This exposes the gauge to damage if the sheet should flutter and also causes misalignment over short periods of time due to the high kinetic energy generated at rolling mill speeds. Also, beta gauges require a signal averaging time of approximately 5 seconds to achieve the required accuracy. This prevents the gauge from generating the required measurements within the time tolerances required in rolling mill operations.
Radioisotope gauges have been employed with moderate success in these applications. However, they have a low radiation intensity resulting in fewer electrons being transmitted. This increases the signal to noise ratio and signal processing times to values where optimum results are not obtainable in the time required. As a result, radioisotope gauges are best utilized under conditions where slow speed control of the measuring process is sufficient.
The majority of the above problems are overcome when the measurement is performed using photons generated by X-ray or gamma ray sources of suitable energy and intensity. The X-ray source is the best method of producing photons since the energy and intensity of the device are adjustable to obtain optimum signal-to-noise ratios. The high intensity beam which is produced allows the X-ray gauge to do signal averaging in approximately 0.01 to 0.005 seconds, thereby meeting the time response requirements necessary for high speed process control. The X-ray gauge is also unaffected by the density of air or other materials in its path. However, conventional techniques using x-ray guages are sensitive to alloy composition variations.
In the typical configuration, the X-ray transmission gauge includes a source of radiation on one side of the sheet material and a detector on the opposite side. The X-ray gauge determines the thickness of the material by measuring the intensity of the radiation which has penetrated the material. From this, the intensity of the radiation which is absorbed by the material can be determined. The intensity of radiation absorbed by an element of the material is determined by its thickness absorption coefficient which is a function of the frequency of the radiation.
It is not uncommon for the sheet materials to contain varying amounts of alloys which have different absorption coefficients than that of the principle sheet material. In the conventional single beam or single energy band radiation thickness gauge, a change in the composition of the material changes the material's overall radiation absorption, which appears as a change in thickness to the instrument.
There have been several attempts to desensitize the X-ray gauges to alloy composition. Most of these have relied on some form of spectral analysis to determine the chemical composition of the metal, and a compensation circuit or algorithm to combine the results of this measurement with the measurement of the X-ray attenuation. To perform a spectral analysis, backscatter techniques have been used by placing a detector at an angle on the same side of the metal to measure the amount of radiation which is reflected off the metal surface. The results of the backscatter system are used to modify the results of the attenuation system. The time required for this compensation technique, however, is not within the tolerances discussed above.
Other systems have used two different sources, each having a different energy level to determine the shift in absorption properties to compensate for the alloy composition. U.S. Pat. No. 4,037,104 to Allport appears to show an X-ray transmission gauge utilizing two energy sources and two detectors. While this device increases the accuracy of the measurements as compared to a single energy source, the additional source and detector are costly. Allport also appears to disclose an alternative embodiment of two detectors stacked in line with each other in the '104 device. The two detectors are positioned so that the X-ray beam penetrates both of them. The sections of the stacked detector are separated by filters to allow only a certain portion of the energy spectrum to pass through the first detection stage and enter the remaining stage. This technique is expensive to implement due to the material and tolerance requirements placed upon the filters. Also, the use of only two radiation sources neglects important alloy elements and is therefore, not as accurate as the proposed invention.
The basis of any spectral analysis is the filtering process that is utilized. For example, some systems have used a multichannel analyzer which converts electric pulses proportional to the X-ray photon's energy into a numeric value used to produce a spectral histogram. This is excellent for small numbers of photons, since the multichannel analyzer is limited to a maximum of approximately 80,000 pulses per second. This small number of photons requires a low flux density radiation source. For a fast response and a low signal-to-noise ratio, however, a large flux density is required. This in turn will result in more photons than the multichannel analyzer can support.
What is needed is a cost effective X-ray transmission device which has the capability to provide measurements of the true thickness of a sheet material during the rolling of a coil, wherein both the thickness and the composition may change independently.